Light in nature covers an enormous range of luminance levels, from starlight to bright sunlight. Yet traditional imaging technologies, both digital and analog, offer diminished experience because they cannot parallel the wide range of luminance and contrast that is visible to human eyes. In response, HDR technologies are being developed to allow for the display of an extended range of color, luminance and contrast. HDR technologies focus on capturing, processing and displaying content of a wider dynamic range. Although there are HDR displays, and although cameras capable of capturing an increased dynamic range are being developed, there is still very limited HDR content available.
To prepare conventional content for HDR displays, reverse or inverse tone mapping operators (ITMO) can be employed in connection with algorithms to process the luminance information in an image with the aim of recovering or recreating the appearance of the image's original scene. Typically, ITMOs take a conventional image as input (e.g., a standard dynamic range (“SDR”) image), expand the luminance range of the image in a non-linear manner, and process highlights or bright regions locally to enhance the HDR appearance of the resulting image.
Existing ITMO solutions focus on perceptually reproducing the appearance of an original scene and rely on strict assumptions about the image's content. Existing ITMO solutions are rarely created with video in mind and they do not provide sufficient controls to be used within a color grading context.
Existing ITMO solutions are problematic because they increase the range of luminance values by a very large amount. The difference between light levels observed in the real world and the input (normally around 100 nits) is very significant, forcing the design of appropriate ITMOs to make trade-offs between visual quality and computational complexity. For example, existing ITMOs enlarge a very low range of legacy non-HDR content (e.g., 100 nits content), into a full luminance channel information (e.g., maximum 4000 nits content).
In some cases, however, the ITMO is required to enlarge the range by a significantly smaller factor, e.g., from a maximum of 1500 nits to 4000 nits (e.g., for range expansion between different HDR displays). This fits with the currently observed trend that display devices have a dynamic range that is increasingly less uniform. With a large variety of displays with different capabilities deployed, content providers will not be able to provide separate grades for each different display.